Seat belt tension sensor having shock isolation

ABSTRACT

A seat belt tension sensor is attached to a seat belt assembly in a vehicle. The seat belt tensor sensor is secured to the vehicle by a bolt. The sensor includes a housing that is secured by the bolt. An anchor plate is mounted to the housing. The anchor plate has a hole for the bolt and an opening. A strain gage is secured to the housing and extends into the opening. The strain gage has an aperture. A spring is located between the anchor plate and the strain gage. An actuator support is located between the spring and the strain gage to actuate the strain gage. The actuator has a projection extending through the aperture that contacts the anchor plate. The projection prevents a shift in the output of the strain gage when the sensor is subjected to mechanical shock.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 09/923,151 filed Aug. 6, 2001 now U.S. Pat. No. 6,578,432, andtitled, “Seat Belt Tension Sensor”.

BACKGROUND OF THE INVENTION

This application is related to the following U.S. patent applications:

U.S. patent application Ser. No. 09/884,615, filed Jun. 19, 2001 andtitled, “Seat Belt Tension Sensor With Overload Protection”.

U.S. patent application Ser. No. 10/202,727, filed Jul. 25, 2002 andtitled, “Hall Effect Seat Belt Tension Sensor”.

U.S. patent application Ser. No. 10/185,242, filed Jun. 28, 2002 andtitled, “Seat Belt Tension Sensor”.

U.S. patent application Ser. No. 09/441,350, filed Nov. 15, 1999 andtitled, “Automobile Seat Having Seat Supporting Brackets with a SteppedWeight Sensor”.

U.S. patent application Ser. No. 09/374,874, filed Aug. 16, 1999 andtitled, “Automobile Seat Weight Sensor”.

U.S. patent application Ser. No. 09/374,870, filed Aug. 16, 1999 andtitled, “Vehicle Occupant Position Detector and Airbag Control System”.

U.S. patent application Ser. No. 09/422,382, filed Oct. 21, 1999 andtitled, “Vehicle Seat Weight Sensor”.

U.S. Pat. No. 6,209,915, issued Apr. 3, 2001 and titled, “Seat BeltTension Sensor”.

U.S. Pat. No. 6,450,534, issued Sep. 17, 2002 and titled, “Seat BeltTension Sensor”.

The foregoing patents have the same assignee as the instant applicationand are herein incorporated by reference in their entirety for relatedand supportive teachings.

1. Field of the Invention

This invention relates to an automobile sensor for detecting themagnitude of a tensile force in a seat belt used in a car seat, and inparticular to a sensor that can detect the magnitude of tension in aseat belt and provide an electrical signal that is representative of themagnitude of tensile force.

2. Description of the Related Art

Air bags have been heralded for their ability to reduce injuries andsave lives. However, since their incorporation into automobiles, aproblem has existed with people of smaller size and small children. Airbags are designed to cushion the impact of occupants and thus reduce theinjuries suffered. However, the force needed to properly cushion theoccupant varies based on the size and position of the person.

For example, a larger person requires the bag to inflate faster and thuswith more force. A smaller person may be injured by a bag inflating atthis higher inflation force. A smaller person is more likely to besitting close to the dashboard and would therefore stand a higher chanceof being injured by the impact of the inflating bag, as opposed to thepassenger hitting the fully inflated bag to absorb the impact of theaccident. An average-sized person can also be injured by an airbaginflation if they are leaning forward, as for example, if they areadjusting the radio.

Because of the concern over injury to passengers in these situations,the National Highway Transportation Safety Administration (or NHTSA), anadministrative agency of the United States, is instituting rulesrequiring the air bag deployment system to identify the passenger sizeand position and inflate the air bag accordingly.

One way to accomplish this task is to use a seat belt tension sensor inconjunction with an occupant weight sensor. The weight sensor canprovide an indication of the force placed by an occupant on the seat.However, if the seat belt is unduly tightened, it can place anadditional downward force on the passenger, creating an erroneous weightreading. Similarly, it is common for infant car seats to be securedtightly to the seat. In this circumstance, it is critical for the systemto recognize that the passenger does not warrant inflation of the airbag. By sensing the tension on the seat belt in addition to the weightreading from the seat, the actual weight of the occupant can bedetermined. This allows for the system to safely deploy the air bag.

SUMMARY OF THE INVENTION

It is a feature of the present invention to provide a seat belt tensionsensor for use with a seat belt assembly. The seat belt tensor sensor issecured to a vehicle by a bolt. The sensor includes a housing that issecured by the bolt. An anchor plate is mounted to the housing. Theanchor plate has a hole for the bolt and an opening. A strain gage issecured to the housing and extends into the opening. The strain gage hasan aperture. A spring is located between the anchor plate and the straingage. An actuator support is located between the spring and the straingage to actuate the strain gage. The actuator has a projection extendingthrough the aperture.

It is a feature of the present invention to provide a seat belt tensionsensor for use with a seat belt assembly. The seat belt tension sensoris secured to a vehicle by a bolt. The sensor includes a housing that issecured by the bolt. The housing has a flange. An anchor plate ismounted to the housing. The anchor plate has a hole for the bolt and anopening. The flange extends into the hole. A strain gage is mounted tothe housing to sense tension in the seat belt assembly and generate anelectrical signal proportional to the amount of tension. A spring ismounted between the strain gage and the anchor plate. The springtransfers a first magnitude of tension from the seat belt assembly tothe strain gage. The anchor plate and the flange cooperate to preventtension greater than the first magnitude from being applied to thestrain gage. Tension greater than the first magnitude is divertedthrough the flange to the bolt. A projection extends into the opening. Arib extends into the opening. The strain gage has a first side adjacentto the rib and a second side adjacent the spring. The spring has a firstand second end. The first end is mounted over the projection. The secondend is adjacent the second side of the strain gage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective exploded view of the preferred embodiment of aseat belt tension sensor.

FIG. 2 is a cross-sectional view of FIG. 1 in an assembled state.

FIG. 3 is another cross-sectional view of FIG. 1 in an assembled state.

FIG. 4 is a perspective exploded view of an alternative embodiment of aseat belt tension sensor.

FIG. 5 is a perspective assembled view of FIG. 4 without the cover.

FIG. 6 is a perspective assembled view of FIG. 4 with the cover.

FIG. 7 is a side view of the strain member.

FIG. 8 is a schematic diagram of the wheatstone bridge circuit.

FIG. 9 is a side view of the seat belt tension sensor of FIG. 5 showingan unloaded state.

FIG. 10 is a side view of the seat belt tension sensor of FIG. 5 showinga fully loaded state.

FIG. 11 is a graph of force versus travel for the seat belt tensorsensor of FIG. 5.

FIG. 12 is an exploded view of an alternative embodiment of a seat belttension sensor.

FIG. 13 is a cross-sectional view of the seat belt tension sensor ofFIG. 12.

FIG. 14 is a perspective view of the seat belt tension sensor of FIG.12.

FIG. 15 is an exploded view of an alternative embodiment of a seat belttension sensor.

FIG. 16 is a top view of the seat belt tension sensor of FIG. 15.

FIG. 17 is a perspective exploded view of the preferred embodiment of aseat belt tension sensor.

FIG. 18 is a top assembled view of FIG. 17 without the cover.

FIG. 19 is a cross-sectional view of FIG. 17 in an assembled state.

FIG. 20 is another cross-sectional view of FIG. 17 in an assembled stateshowing the elastomeric washers.

FIG. 21 is a perspective assembled view of FIG. 17 FIG. 22 is a view ofFIG. 20 with the seat belt and bolt added.

FIG. 23 is a front view of the strain gage.

FIG. 24 is a schematic diagram of a wheatstone bridge.

FIG. 25 is a graph of force versus deflection for the seat belt tensionsensor of FIG. 17 and a series of diagrams showing how the sensor moveswith applied tension.

It is noted that the drawings of the invention are not to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a seat belt tension sensor. Referring to FIGS.1, 2 and 3 there is a seat belt tension sensor assembly 10 shown.Assembly 10 has a limit structure 11 and a sensor 21. Limit structure 11is fastened between seat belt components. For example, structure 11 canbe attached to an anchor plate 46 that is attached to a seat beltwebbing 52 and to a vehicle seat or floor 8 (FIG. 2) by fastener or bolt54.

Limit structure 11 has a housing 12 with a cavity 14. A cylindricalflange 16 extends into cavity 14 and includes a hollow center portion20. A recess 18 is provided on one side of housing 12 to allow anchorplate 46 to extend beyond housing 12. A circular hub 22 has a flat discshaped portion 23, and a cylindrical rim 25 attached to and extendingfrom the disc shaped portion 23. The rim 25 has an aperture 28 passingtherethrough. Several slots 24 are located on disc 23. Several screwholes 26 are also located in disc 23. Disc 23 has several arms 27 thateach have a strain gage resistor 29 mounted thereon. Hub 22 goes overflange 16 and creates a gap 17 between the rim and flange as shown inFIG. 2.

A cover 38 goes over housing 12 and is retained to housing 12 by screws44 or other fasteners that pass through screw holes 26 and into threadedbores 13 in housing 12. Cover 38 has screw holes 40 and a centralopening 42. Several springs 36 are located between cover 38 and discportion 23. Anchor plate 46 has a mounting hole 48 that goes over rim25. Plate 46 has a slot 50 that the seat belt webbing 52 passes through.Webbing 52 would typically be looped through slot 50 and then sewed ontoitself to securely fasten the webbing to the plate.

After assembly, threaded fastener or bolt 54 extends through hollowcenter portion 20, rim aperture 28, central opening 42 and mounting hole48 and mates with threads in a vehicle floor or seat 8. Fastener 54 is aconventional fastener such as a hex headed bolt, Allen head screw,shoulder screw or rivet.

Sensor 21 includes the hub 22, disc portion 23 and rim 25. Hub 22 ispreferably formed from 430 stainless steel. Several strain gageresistors 29 are arranged on arms 27 around rim 25. Details of theconstruction and operation of resistors 29 are shown in U.S. patentapplication Ser. No. 09/441,350, filed Nov. 15, 1999 and titled,“Automobile Seat Having Seat Supporting Brackets with a Stepped WeightSensor”.

Resistors 29 would typically be connected in a conventional wheatstonebridge configuration. Resistors 29 are strain sensitive and will changeresistance based on the amount of strain in arms 27. A wiring harness 31would typically connect resistors 29 to an external electrical circuit.

Referring to FIG. 2, showing a normal load state for the presentinvention, when an occupant sits in a vehicle seat and buckles a seatbelt, the tension placed in the seat belt is transferred from webbing 52to buckle plate 46 to sensor 21 through rim 25 and disc 23. The force istransferred from disc 23 through springs 36 to cover 38 and housing 12then to floor 8 through fastener 54. In this state, two of springs 36are being compressed pressing down on disc 23 while two of springs 36are in tension. An electrical output signal is generated by theresistors 29 that is proportional to the magnitude of the tension in theseat belt and is transmitted over a wire harness 31 to a conventionalair bag controller (not shown). The air bag controller can then use theseat belt tension information to compute a more accurate profile of theseat occupant and use that information to control deployment of theairbag. This is the normal operational state of the seat belt tensionsensor in which all of the seat belt tension is carried through thesensor 21.

In a situation where the vehicle is involved in a crash, the seat belttension sensor operates in a different mode called a high load or crashstate as shown in FIG. 3. In the high load state, the limit structurecarries the tension placed on the seat belt. The amount of tension inthe seat belt in a crash situation is much larger than in normaloperation. If the hub 22 was designed to carry all of this tension, itwould not flex enough to properly function as a strain gage sensor.Therefore, in a crash situation, the limit structure carries the tensionthrough the much stronger limit structure 11. As the tension in the seatbelt increases, the hub 22 rotates or tilts until rim 25 contacts flange16 providing a positive stop. The seat belt tension is then transferredthrough buckle plate 46 to rim 25, flange 16, bolt 54 and then to thevehicle floor or seat frame 8. The limit structure 11 is designed so asnot to deform under the load placed on it by the seat occupant during acrash situation.

REMARKS ABOUT THE PREFERRED EMBODIMENT

The seat belt tension sensor has several advantages. It allows accuratesensing of seat belt tension, while at the same time providing thestructural strength needed for occupant restraint in a crash situation.The seat belt tension sensor allows an airbag controller to make betterdecisions as to when and how to deploy and airbag based upon moreaccurate seat occupant information. In the case of a child's car seatbeing strapped into a car seat, the seat belt tension sensor inconjunction with a seat weight sensor allows the airbag controller toproperly compute that the seat occupant has a low weight and to preventdeployment of the airbag.

The gap between the hub and the housing flange is the travel range ofthe sensor as it is actuated. This design solves four main problems. 1)Maintaining sensitivity at low loads without damage at higher loads. 2)Maintaining restraint system integrity. 3) Integration into multiplerestraint systems. 4) Reading correctly over a wide range of strainangles. A theta angle represents rotation about the axis of the bolt. Analpha angle represents tilt toward and away from the seat.

1) Maintaining Sensitivity at Low Loads Without Damage at Higher Loads.

When the gap between the rim and the flange is closed the load appliedto the strain sensor elements reaches its limit. After this, the load istransferred to the bolt. Limiting the maximum load applied to the strainsensor is necessary since the working range of the sensor is generallybelow 100-lbs. but the sensor must withstand large (often greater than1000-lb.) loads without damage and must not compromise the integrity ofthe passenger restraint system.

Springs 36 can be omitted if desired. Springs 36 are added to allowlarger motions. These springs apply a load, which holds the hub 22 inplace until a designed actuation force is reached. Once this force isexceeded the springs allow the hub to move. This both limits the loadapplied and allows the tolerances between the limit structure 11 and theshoulder bolt 54 to be looser.

2) Maintaining Restraint System Integrity.

The present design allows the use of the same or very similar mountingbolts and anchors and mounting technique as do current seatbeltattachment methods. Thus, safety engineers are very familiar with therequirements of the attachment method and installation procedures arechanged as little as possible.

3) Integration into Multiple Restraint Systems.

This present invention allows the sensor to be attached at the mostcommon point of a wide variety of belt systems. It is useable even withvery short bolt to buckle distances. The only changes required in someinstallations are a larger mounting hole in the anchor and a longer boltto allow for the thickness of the device. The shoulder bolt is used asthe stop. A tube through which the bolt passes could also be used as astop. The advantage of this design is that it does not require ashoulder bolt with the correct shoulder length and diameter for thedevice to operate. A standard bolt may be used. This improves fieldserviceability.

4) Reading Correctly Over a Wide Range of Theta and Alpha Angles.

Theta represents rotation about the axis of the bolt. Alpha representstilt toward and away from the seat. The attachment to the floor may befixed or, more likely, the sensor would be free to rotate and tilt plusor minus several degrees. This freedom of movement allows the sensor torespond only to tension on the seatbelt while accommodating user needs.The cross-style sensor design allows the tension load to be applied at afairly wide range of theta angles with only small changes insensitivity. This allows the sensor to rotate (theta) normally toaccommodate passenger needs. If desired, the sensor can rotate aroundthe bolt axis in order that its sensing elements could remain alignedwith the direction of the pull throughout the normal rotation limit ofthe anchor. This could allow the use of simplified sensor elementdesigns.

The sensor 21 may also be shaped such that only tension is sensed andcompression is ignored. This is accomplished by removing one or morearms 27 on the “top” side of the sensor. In the event that the angle oftension application is desired, the sensor can be constructed so thatthe alpha angle is reported as if it were tension. Or, another separatestrain gage or potentiometric sensors or the like can be used to reportthe angle(s) (alpha and/or theta) at which the tension load is applied.

FIRST ALTERNATIVE EMBODIMENT

Referring to FIGS. 4–6 there is an alternative embodiment of a seat belttension sensor assembly 200 shown. Assembly 200 comprises a limitstructure 200A and a sensor 200B. Limit structure 200A includes ahousing 202, an anchor plate 210, a cover 250, and a strain sensor ormember 220. Housing 202 has an aperture 201, flange 203, spring channel204, walls 206 and 207 that define a strain member slot 205 and holes208. Anchor plate 210 is loosely fitted within housing 202. Anchor plate210 includes an arm 212 extending between a first cutout 213 and asecond cutout 214. Seat belt webbing 290 is attached through cutout 214.A pair of projections 215 and 216 extend into first cutout 213. Amounting hole 217 is provided in anchor plate 210. A spring 218 ismounted in channel 204. One end of spring 218 is mounted over projection215. Sensor 200B has a strain member 220 that is mounted in slot 205. Asupport 219 fits into spring 218. Support 219 rests adjacent a surfaceof strain member 220 and serves to even out the forces from spring 218on strain member 220.

A wire harness 230 has several wires 232 that end in terminals 234.Terminals 234 clip over connector pads 226 on member 220. A cover 250has a hole 252 and projections 254. Projections 254 mate with holes 208to snap fit cover 250 to housing 202. Seat belt tension sensor 200 isattached to a vehicle floor (not shown) by a fastener 280 such as abolt. Fastener 280 goes through holes 201, 217 and 252.

Strain member 220, shown in FIGS. 7 and 8, is formed of a materialcapable of carrying the tension applied by spring 218 when the seat beltis tightened. Preferably, the strain member 220 is formed of 430stainless steel. The strain member 220 includes stain sensitiveresistors 222 a,b,c,d formed thereon. These are formed by firstscreening a dielectric layer 224 onto the steel. The strain member 220is then kiln fired at 850° C. Next, electrically conductive traces 225and connector pads 226 a,b,c,d are similarly screened onto the strainmember 220. The strain member 220 is again kiln fired at 850° C. Thestrain sensitive resistors 222 a,b,c,d are next screened onto the strainmember 220 in positions defined by the electrically conductive traces225. The strain member 220 is again kiln fired at 850° C. At this point,a final coating of a covercoat or epoxy can be applied to protect theelectrical components of strain member 220. This coating is notrequired, but may be desirable in circumstances where high abrasion orcontaminants are expected. It should be noted that the strain sensitiveresistors 222 a,b,c,d and connector pads 226 a,b,c,d together form theWheatstone bridge circuit of FIG. 8.

Turning to FIGS. 4–5, and 9–10, when a tension is applied to seat belt290, anchor plate 210 causes spring 218 to apply pressure to the centerof the strain member 220. As the tension increases, the strain sensitiveresistors 222 will change resistance resulting in an electrical outputsignal that changes in proportion to the amount of strain in seat belt290. This electrical signal is provided to an external electricalcircuit by wire harness 230.

In a collision situation, the force applied to the seat belt overcomesthe spring resistance and anchor plate 210 moves to rest against flange203. Fastener 280 passes through hole 201 and is adjacent to flange 203.The force from the seat belt is transferred to fastener 280 which isattached to a vehicle floor. Thus, the force is transferred form theseat belt to the vehicle floor. In this way, no further tension isapplied to the strain member 220 and the strain member 220 is thusprotected from excessive forces by limit structure 220A.

A graph showing force versus travel for sensor 200 is provided in FIG.11. Note that the graph varies linearly up to the point of overtravel atwhich point it flattens indicating and overtravel condition. The outputlimit is set for the point at which the fastener 280 engages against theanchor plate 210.

SECOND ALTERNATIVE EMBODIMENT

FIGS. 12–14 show another seat belt tension sensor assembly 300. Assembly300 comprises a limit structure 302 and a sensor 304. Limit structure302 includes a housing 310, an anchor plate 330, an actuator pin 340 andbolt 360. Housing 310 has an aperture 312, a channel 314, rails 318 andstrain member slot 316. Anchor plate 330 is slidably retained withinhousing 310 by rail 318. Anchor plate 330 includes a cutout 336, a hole338 and a mounting hole 332. Seat belt webbing would be attached throughcutout 336. A pair of springs 342 are mounted in slot 316. Sensor 304has a strain member 320 with resistors 322 that are mounted in slot 316.Actuator pin 340 rests against the backside of strain member 320. A pairof foam washers 350 are included in the installation of the sensor toreduce vibration noise and allow axial off set loads. The foam washersallow the seat belt tension sensor to flex slightly depending on thedirection of pull on the seat belt webbing.

Seat belt tension sensor 300 is attached to a vehicle floor or seat (notshown) by a fastener 360 such as a bolt. Fastener 360 goes through holes312 and 332.

Strain member 320 is similar to strain member 220, shown in FIGS. 7 and8. When a tension is applied to the seat belt webbing, anchor plate 330causes pin 340 to apply pressure to the center of the strain member 320.As the tension increases, the strain sensitive resistors 322 will changeresistance resulting in an electrical output signal that changes inproportion to the amount of strain in the seat belt. This electricalsignal is provided to an external electrical circuit by a wire harness(not shown).

In a collision situation, the force applied to the seat belt overcomesthe spring resistance and anchor plate 330 moves to rest against bolt360. The force is transferred form the seat belt to the vehicle floor.In this way, no further tension is applied to the strain member 320 andthe strain member 320 is thus protected from excessive forces by limitstructure 302.

THIRD ALTERNATIVE EMBODIMENT

FIGS. 15–16 show another seat belt tension sensor assembly 400. Assembly400 comprises a limit structure 402 and a sensor 404. Limit structure402 includes a housing 410, a anchor plate 430, an actuator bar 440,cover 450 and bolt 460. Housing 410 has an aperture 412 and rails 414.Anchor plate 430 is slidably retained within housing 410 by rails 414.Anchor plate 430 includes a first cutout 436, a second cutout 434, apair of projections 438 and a mounting hole 432. Seat belt webbing wouldbe attached through cutout 436. A pair of springs 446 are mounted inover projections 438. Sensor 404 has a strain member 420 with resistors422 that are mounted in cutout 434. Actuator 440 has a bump 442 thatrests against the backside of strain member 420. Several fingers 416 areattached to housing 410 around hole 415. The fingers grip aroundfastener 460 and allow axial off set loads. The fingers 416 allow theseat belt tension sensor to flex slightly depending on the direction ofpull on the seat belt webbing.

Seat belt tension sensor 400 is attached to a vehicle floor or seat (notshown) by a fastener 460 such as a bolt. Fastener 460 goes through holes415 and 432.

Strain member 420 is similar to strain member 220, shown in FIGS. 7 and8. When a tension is applied to the seat belt webbing, anchor plate 430compresses spring 446 to apply pressure to actuator 440 that allows bump442 to apply pressure to the center of the strain member 420. As thetension increases, the strain sensitive resistors 422 will changeresistance resulting in an electrical output signal that changes inproportion to the amount of strain in the seat belt. This electricalsignal is provided to an external electrical circuit by a wire harness(not shown).

In a collision situation, the force applied to the seat belt overcomesthe spring resistance and anchor plate 430 moves to rest against bolt460. The force is transferred form the seat belt to the vehicle floor.In this way, no further tension is applied to the strain member 420 andthe strain member 420 is thus protected from excessive forces by limitstructure 402.

VARIATIONS OF THE PREFERRED EMBODIMENT

The sensor shown was several strain gage resistors, one skilled in theart will realize that the preferred embodiment would work with othertypes of sensors. For example, discrete chip resistors could be attachedor foil type strain gages could be used. Furthermore, the shape of thesensor could be varied to any configuration that would transfer theweight from the seat belt.

Another variation of the seat belt tension sensor would be to utilizeother electrical connections other than a wire harness. For example, aconnector or terminals could be added.

Yet, a further variation, would be to place signal conditioningcircuitry on hub 22 or strain member 200 to amplify and filter theelectrical signal before it is transmitted to the airbag controller.

The seat belt tension sensor shown was mounted between a seat belt and avehicle member. One skilled in the art will realize that the preferredembodiment could be mounted to various locations on the seat or vehicleinterior. For example, the seat belt tension sensor could be attached tothe vehicle roof.

The illustrated embodiment showed the use of the seat belt tensionsensor in an automobile seat. It is contemplated to utilize the seatbelt tension sensor in other occupant sensing applications such aschairs, sofas, scales, beds and mattresses, hospital equipment, cribs,airplane seats, train seats, boat seats, amusement rides, and theaterseats.

FOURTH ALTERNATIVE EMBODIMENT

Referring to FIGS. 17–25, there is a seat belt tension sensor assembly800 shown. Assembly 800 comprises a limit structure 802A and a sensor802B. Limit structure 802A includes a housing 202, an anchor plate 210and a cover 250. Housing 202 has an aperture 201, flange 203, walls 206and 207 that define a strain gage slot 205, posts 840, recess 904 and areverse stop 902. Housing 202 is preferably molded from plastic. Anchorplate 210 is loosely fitted within housing 202. Anchor plate 210 hasends 210A and 210B. Anchor plate 210 includes an arm 212 extendingbetween a cutout 213 and a slot 214. Seat belt webbing 290 is attachedthrough slot 214. A projection 216 extends into cutout 213. A rib 906extends into cutout 213 opposite to projection 216. A mounting hole 217is provided in anchor plate 210. A spring 218 is mounted in cutout 213between strain gage 850 and projection 216. Spring 218 has ends 218A and218B. End 218A is mounted over projection 216.

Sensor 802B has a strain gage 850. Strain gage 850 has a substrate 852that is mounted in slot 205. Substrate 852 has surfaces 852A and 852B.Substrate 852 has an aperture 856 that extends through the strain memberand a cover coat 854. An actuator 219 fits into spring end 218B.Actuator 219 has a projection 219A and an end 219B. End 219B fits intospring end 218B. Projection 219A extends through aperture 856 of strainmember 852. Projection 219A contacts rib 906. The spring 218 ispre-loaded or compressed during installation to a spring force of 35pounds.

A wire harness 230 has several wires 232 that are connected to anelectronic circuit assembly or printed circuit board 844. A connector842 is connected to one end of wire harness 230. Wire harness 230 restsin recess 904 as it exits housing 202. Terminals 858 are soldered toconnector pads 226 on strain gage 852. The other end of the terminalsare soldered into printed circuit board 844. Printed circuit board 844has electronic components such as signal conditioning electronics 846mounted to it. The electronic circuit assembly takes the signal from thestrain gage and filters and amplifies it before it is transmitted overthe wire harness to an airbag controller or another electronic module.

Cover 250 has a hole 252 and recess 816. Cover 250 is ultrasonicallywelded to housing 202. An annular spring ring or washer 810 fits intorecess 816. The spring ring 810 has several tines 812 that extendupwardly. Tangs 814 hold spring ring 810 in recess 816. Spring ring 810reduces rattling and movement of the housing about the bolt 280 afterthe sensor is mounted. Spring ring 810 reduces noise in the vehicleinterior by biasing the housing away from the bolt head toward themounting surface.

A cable guide 826 is positioned over a portion of end 210B. Cable guide826 holds wire harness 230 away from webbing 290. Cable guide 826 has atab 830 that folds over wire harness 230. Holes 832 mate with posts 828that are heat staked to hold tab 830 in place. The curved portion 834fits over and partially covers end 210B. A seal or shield 820 slips overend 210B and rests against housing 202 and cover 250. Seal 820 has aslot 822 that the anchor plate 210 extends through. Seal 820 preventscontamination and liquids from entering the sensor. Cover 250 has afinger 920.

Posts 840 extend upwardly from housing 202 and downwardly from cover250. A silicone or rubber grommet or washer 838 is mounted on each post840. Washer 838 is located adjacent and in contact with anchor plate210. Washer 838 prevents vibration and rattling of the anchor plate inthe housing.

The seat belt tension sensor 800 is attached to a vehicle structure suchas a seat or floor or B pillar (not shown) by a fastener 280 such as abolt. Fastener 280 goes through holes 252, 217 and 201. Seat beltwebbing 290 loops through slot 214 and is sewn in place. Webbing 290would continue on to wrap around the waist of a vehicle seat occupant ora child seat.

Strain gage 850 is shown in further detail in FIGS. 23 and 24.Preferably, the substrate 852 is formed of 430 stainless steel. Thestrain gage 850 includes strain sensitive resistors 220 a,b,c,d formedthereon. These are formed by first screening a dielectric layer 224 ontothe steel. The substrate is then kiln fired at 850° C. Next,electrically conductive traces 225 and connector pads 226 a,b,c,d aresimilarly screened onto substrate 852 and kiln fired at 850° C. Thestrain sensitive resistors 220 a,b,c,d are next screened onto substrate852 in positions defined by the electrically conductive traces 225 andkiln fired at 850° C. At this point, a final coating of a covercoat orepoxy can be applied to protect the electrical components. This coatingis not required, but may be desirable in circumstances where highabrasion or contaminants are expected. It should be noted that thestrain sensitive resistors 220 a,b,c,d and connector pads 226 a,b,c,dtogether form the Wheatstone bridge circuit of FIG. 24.

When a tension is applied to seat belt 290, anchor plate 210 causesspring 218 to apply force to the center of the substrate 852. As thetension increases, the strain sensitive resistors 220 will changeresistance resulting in an electrical output signal that changes inproportion to the amount of tension in seat belt 290. This electricalsignal is amplified and conditioned by electronic circuitry 844 and isthen provided to an external electrical circuit such as an airbagcontroller by wire harness 230.

Details of the construction and operation of resistors 220 are shown inU.S. patent application Ser. No. 09/441,350, filed Nov. 15, 1999 andtitled, “Automobile Seat Having Seat Supporting Brackets with a SteppedWeight Sensor”.

The resistors are typically connected in a conventional wheatstonebridge configuration. Resistors 220 are strain sensitive and will changeresistance based on the amount of strain placed on substrate 852.

The actuator projection 219A is in contact with rib 906 preventingdirect contact of the rib to surface 852B. During a shock incident, suchas when the sensor is dropped prior to installation, the projection 219Alocated in aperture 856 allows the anchor plate 210 to move away fromthe seat belt.

Another feature that enhances the shock resistance of sensor 800 is thereverse stop 902 mounted to flange 203. When anchor plate 210 movesbackwardly (towards strain gage 850), reverse stop 902 engages anchorplate arm 212 preventing further backwards movement of plate 210. Thisassists in keeping rib 906 away from substrate surface 852B. The reversestop 902 prevents the anchor plate from moving backwards before it hitsthe strain gage. The reverse stop 902 prevents the backward load fromreaching the sensor 850 and protects against shock damage.

The use of projection 219 allows the reverse stop 902 to contact arm 212before rib 906 hits strain gage 850 therefore preventing damage duringthe shock incident. In this manner the strain gage 850 is isolated fromthe rib 906.

In a high load situation, the force applied to the seat belt overcomesthe force of spring 218. Then, anchor plate 210 at rib 906 moves to restagainst flange 203. Fastener 280 passes through holes 201, 217, 252 andis adjacent to flange 203. Additional force from the seat belt is thentransferred to fastener 280 which is attached directly or indirectly toa vehicle mounting point such as the seat or floor. Thus, the additionalor overload force is transferred from the seat belt to the vehicle. Inthis way, no further tension is applied to the strain gage 850 and it isthus protected from excessive forces by the limit structure 802A. Whenno seat belt tension is applied to anchor plate 210, the spring iscompressed to 35 pounds. When rib 906 engages flange 203, the spring iscompressed to 38 pounds. Any load beyond the 38 pounds is divertedthrough the flange 203 to bolt 280.

A graph showing force versus travel for sensor 800 is provided in FIG.25. FIG. 25 shows the force versus travel measured at three differentlocations:

1. The force applied to the sensor on anchor plate 210;

2. The compression spring force on spring 218;

3. The force on strain gage 850;

The applied force is shown increasing from 0 to 38 pounds. The appliedforce greater than 38 pounds is diverted through flange 203 to bolt 280.The force on spring 218 starts at the pre-compression value of 35 poundsand increases to a maximum of 38 pounds. The force on the strain gage850 starts at 0 pounds and increases to a maximum of 38 pounds. There isno force applied to the strain gage when no tension is applied on theseat belt webbing.

Note that the graph varies linearly in a sensing range from 0 to 30pounds. The sensing range increases up to the point of disengagementwhere projection 219A separates from contact with rib 906. The point ofdisengagement is about 35 pounds plus or minus a few pounds. Between 30and 38 pounds of force the sensor is in the overload range. The curveflattens indicating an overload condition. The limit stop is the pointat which the flange 203 contacts rib 906. Beyond the stop point alladditional force is transferred to fastener 280.

REMARKS ABOUT THE FOURTH ALTERNATIVE EMBODIMENT

The present seat belt tension sensor has several advantages. It providesfor isolation from shock events and protects the strain sensitiveresistors. The use of the actuator projection extending through thestrain gage prevents the rib of the anchor plate from contacting theback side of the strain gage. This protects the strain gage from damageduring a shock incident. The use of the posts and elastomeric washersprevents rattling of the sensor and allows for the sensor to be morequiet in a vehicle. The seat belt tension sensor allows accurate sensingof seat belt tension, while at the same time providing the structuralstrength needed for occupant restraint in a crash situation. The seatbelt tension sensor further protects the strain gage from excessiveloads that could damage the strain gage. The present invention allows anairbag controller to make better decisions as to when and how to deployan airbag based upon more accurate seat occupant information.

While the invention has been taught with specific reference to theseembodiments, someone skilled in the art will recognize that changes canbe made in form and detail without departing from the spirit and thescope of the invention. The described embodiments are to be consideredin all respects only as illustrative and not restrictive. The scope ofthe invention is, therefore, indicated by the appended claims ratherthan by the foregoing description. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1. A seat belt tension sensor for use with a seat belt assembly, theseat belt tension sensor secured to a vehicle by a fastener, the sensorcomprising: a housing adapted to be secured by the fastener; an anchorplate mounted partially in the housing, the anchor plate having a holefor the fastener and an opening; a strain gage secured to the housingand extending into the opening; a spring located between the anchorplate and the strain gage; and an actuator located between the springand the strain gage to actuate the strain gage.
 2. The seat belt tensionsensor according to claim 1 wherein the spring has a predeterminedmagnitude of compression when no tension is applied to the seat beltassembly.
 3. The seat belt tension sensor according to claim 1 whereinthe anchor plate has a projection extending into the opening and a ribextending into the opening, the projection located opposite the rib. 4.The seat belt tension sensor according to claim 3 wherein the straingage has a first and second surface.
 5. The seat belt tension sensoraccording to claim 4 wherein the rib is located adjacent the secondsurface and the actuator is located adjacent the first surface.
 6. Theseat belt tension sensor according to claim 5 wherein the strain gagehas an aperture extending therethrough and the actuator has a projectionthat extends through the aperture and contacts the rib.
 7. The seat belttension sensor according to claim 1 wherein the housing has a postextending therefrom, a washer mounted around the housing such thatvibration is reduced between the housing and the anchor plate.
 8. Theseat belt tension sensor according to claim 1 wherein the anchor platehas a slot for securing a seat belt webbing.
 9. The seat belt tensionsensor according to claim 8 wherein a wire guide is attached to theanchor plate.
 10. The seat belt tension sensor according to claim 1wherein a seal is attached between the anchor plate and the housing. 11.The seat belt tension sensor according to claim 1 wherein the housinghas a recess, a spring washer mounted in the recess.
 12. A seat belttension sensor for use with a seat belt assembly, the seat belt tensionsensor secured to a vehicle by a fastener, the sensor comprising: ahousing adapted to be secured by the fastener; an anchor plate mountedto the housing, the anchor plate having a hole for the fastener and anopening; a projection extending into the opening; a rib extending intothe opening; a strain gage secured to the housing and mounted betweenthe projection and the rib; and a spring located between the projectionand the strain gage, the spring applying a first force to a first sideof the strain gage.
 13. The seat belt tension sensor according to claim12 wherein an actuator is located between the spring and the first side.14. The seat belt tension sensor according to claim 13 wherein thestrain gage has an aperture.
 15. The seat belt tension sensor accordingto claim 14 wherein the actuator has a projection, the projectionextending through the aperture and contacting the rib such that thestrain gage is isolated from an applied shock load.
 16. The seat belttension sensor according to claim 12 wherein the housing has a postextending toward the anchor plate, a washer mounted around the housingsuch that vibration is reduced between the housing and the anchor plate.17. The seat belt tension sensor according to claim 12 wherein thestrain gage further comprises: a substrate; a plurality of resistorsmounted to substrate, the resistors adapted to generate an electricalsignal in response to the strain gage being subjected to an increasednet force, the electrical signal changing as a function of tension onthe seat belt assembly.
 18. The seat belt tension sensor according toclaim 12 wherein the housing and the anchor plate cooperate to limit themaximum net force applied to the strain gage.
 19. A seat belt tensionsensor for use with a seat belt assembly, the seat belt tension sensorsecured to a vehicle by a bolt, the sensor comprising: a housing adaptedto be secured by the bolt, the housing having a flange extendingtherefrom; an anchor plate mounted in the housing, the anchor platehaving a hole for the bolt and an opening, the flange extending into thehole; a strain gage mounted to the housing for sensing tension in theseat belt assembly and generating an electrical signal proportional tothe amount of tension; a spring mounted between the strain gage and theanchor plate, the spring transferring a first magnitude of tension fromthe seat belt assembly to the strain gage; and the anchor plate and theflange cooperating to prevent tension greater than the first magnitudefrom being applied to the strain gage, tension greater than the firstmagnitude being diverted through the flange to the bolt.
 20. The seatbelt tension sensor according to claim 19, further comprising: aprojection extending into the opening; a rib extending into the opening;the strain gage having a first side adjacent to the rib and a secondside adjacent the spring; the spring having a first and second end, thefirst end mounted over the projection, the second end adjacent thesecond side of the strain gage.
 21. The seat belt tension sensoraccording to claim 20, wherein the spring force is balanced by the forceon the rib such that a net force on the strain gage under no appliedseat belt tension is zero.
 22. The seat belt tension sensor according toclaim 20, wherein an actuator is mounted between the spring and thesecond side of the strain gage.
 23. The seat belt tension sensoraccording to claim 22 wherein the strain gage has an aperture.
 24. Theseat belt tension sensor according to claim 23 wherein the actuator hasa projection, the projection extending through the aperture andcontacting the rib such that the strain gage is isolated from an appliedshock load.
 25. The seat belt tension sensor according to claim 19wherein a washer is mounted between the housing and the anchor platesuch that vibration is reduced between the housing and the anchor plate.26. The seat belt tension sensor according to claim 19, wherein theanchor plate further defines a slot for securing a seat belt webbing.27. The seat belt tension sensor according to claim 19 wherein thestrain gage further comprises: a substrate; a plurality of resistorsmounted to substrate.
 28. The seat belt tension sensor according toclaim 27 wherein signal conditioning electronics are mounted in thehousing and are electrically connected to the resistors.
 29. The seatbelt tension sensor according to claim 28 wherein the signalconditioning electronics are mounted on a printed circuit board.
 30. Theseat belt tension sensor according to claim 29 wherein a terminalconnects the printed circuit board to the resistors.
 31. The seat belttension sensor according to claim 29 wherein a wire harness is connectedto the printed circuit board and emanates from the housing.